Field of the Invention
The present invention relates to carbon molecular sieve membranes and gas separations utilizing the same.
Related Art
Membranes are often preferred to other gas separation techniques in industry due to the following advantages. The energy consumption for membranes is low as they do not require a phase change for separation. Membrane modules are compact, thereby reducing their footprint and capital cost. Membranes are also mechanically robust and reliable because they have no moving parts.
Polymer membranes in particular are used in a wide variety of industrial applications. They enable the production of enriched nitrogen from air. They separate hydrogen from other gases in refineries. They are also used to remove carbon dioxide from natural gas.
However, owing to the manufacturing processes and material structure, today's polymeric membranes cannot reach both high selectivities and permeabilities, because a trade-off exists between permeability and selectivity. Robeson formulated semi-empirical upper-bound trade-off lines for several gas pairs. (Robeson, “The upper bound revisited”, Journal of Membrane Science 2008, vol 320, pp 390-400 (2008)).
Carbon molecular sieve (CMS) membranes have been shown to exceed the Robeson upper-bound and therefore are quite promising for use in gas separation membranes. CMS membranes are produced by pyrolyzing the precursor polymeric membranes to leave an amorphous carbon framework containing a network of micropores and ultramicropores. CMS membranes are considered molecular sieves because, when formed in an appropriate manner, the ultramicropores have dimensions that are sized to discriminate between pairs of gas molecules having similar kinetic diameters (such as O2/N2, CO2/N2, and CO2/CH4). In other words, slightly smaller gas molecules may be separated from slightly larger gas molecules by the presence of the appropriately sized ultramicropores.
Despite the very promising data shown so far, there still remains a need for CMS membranes exhibiting more satisfactory performance (i.e., permeabilities and selectivities for typical gases of interest). While some have proposed materials or techniques for producing CMS membranes having relatively high permeabilities, the selectivities for common gas pairs (such as O2/N2, H2/N2, CO2/CH4, CO2/N2, etc.) are not wholly satisfactory. Some have theorized that such CMS membranes are too porous. Conversely, while some have proposed materials or techniques for producing CMS membranes having relatively high selectivities, their permeabilities are similarly not wholly satisfactory. Some have theorized that such CMS membranes are too dense.
Some have proposed that the pyrolysis atmosphere may play a part in determining the result permeability or selectivity exhibited by a CMS membrane.
In particular, some have disclosed pyrolysis of precursor polymeric membrane under a CO2 atmosphere but no discussion was made with regard to the effect of the CO2 atmosphere upon the resultant CMS membrane. For example, Campo, et al. disclosed the pyrolysis of cellophane paper precursor membranes at various pyrolysis soak temperatures and soak times and under various pyrolysis atmospheres (Carbon molecular sieve membranes from cellophane paper, Journal of Membrane Science 350 (2010) 180-188). While cellophane paper membranes were pyrolyzed under 99.999% pure N2, Ar, and CO2, only the permeabilities and selectivities for the N2 pyrolysis were reported. Therefore, no comparison between the effects of pyrolysis atmosphere composition upon the permeability and selectivity the resultant CMS membrane can be made.
Others have studied the pyrolysis of precursor polymeric membranes under vacuum and different types of inert gases. For example, Su, et al. disclosed the pyrolysis of Kapton polyimide membranes under vacuum, under Ar, under He, and under N2 at different temperatures (Effects of carbonisation atmosphere on the structural characteristics and transport properties of carbon membranes prepared from Kapton® polyimide, Journal of Membrane Science 305 (2007) 263-270). They found that pyrolysis under He resulted in the highest BET surface area, total pore volume and micropore volume. They also found that the differences in permeance between the inert gas atmospheres were only significant at lower pyrolysis temperatures of 600° C. Moreover, they found that the highest ideal selectivity for O2/N2 (as opposed to mixed gas selectivity) was produced by pyrolysis under vacuum.
Still others have studied the pyrolysis of precursor polymeric membranes under vacuum and while being purged with inert gas, including Ar, He, and CO2. Geiszler, et al. disclosed the pyrolysis of BPDA:6FDA/DAD under Ar, He, CO2, and vacuum at varying soak temperatures and inert gas flow rates (Effect of polyimide pyrolysis conditions on carbon molecular sieve membrane properties, Industrial Engineering Chemical Research 35 (1996) 2999). They found that, for a given pyrolysis temperature, vacuum pyrolysis produced higher O2/N2 and H2/N2 selectivities than did pyrolysis carried out under an inert gas purge of Ar, He, or CO2. At a soak temperature of 550° C. and an inert gas purge flow rate of 200 cm3(STP)/min, they found little difference in the O2 flux and O2/N2 selectivity for membranes pyrolyzed with either an Ar, He, or CO2 inert gas purge. They disclosed that CO2 becomes more oxidative and pyrolyzed membranes with a CO2 gas purge with an 800° C. soak temperature. While such pyrolysis conditions produced a CMS membrane having a relatively high flux of about 6500 GPU (gas production units), it exhibited a very poor O2/N2 selectivity of about 1.0.
Finally, others have also studied the oxidation of CMS membranes using either O2 or CO2. Hayashi, et al. disclosed the pyrolysis of precursor polymeric membranes under deoxygenated N2 at 600-800° C. followed by oxidation with N2/O2 at 300° C. and pyrolysis of precursor polymeric membranes under deoxygenated N2 at 900° C. followed by oxidation at the same temperature with CO2 (Effect of Oxidation on Gas Permeation of Carbon Molecular Sieving Membranes Based on BPDA-pp'ODA Polyimide, Industrial Engineering Chemistry Research (1997), 36, 2134-2140). Oxidation at 900 C for 1-3 hours resulted in either partial or total peeling of the membrane from the porous alumina support tube. Otherwise, CO2 oxidation for 1 h at 800° C. or for 5 min at 900° C. had no effect on permeance. Additionally, excess oxidation abruptly expanded the pore size and decreased permselectivities for permeants larger than 0.4 nm. The researchers concluded that the control of micropore size was not achieved by CO2 oxidation at 800-900° C.
Apart from the difficulty achieving an initially desirable performance, CMS membranes also exhibit performance degradation over time. This is believed to be caused by two mechanisms. The first mechanism is physical in nature. Similar to glassy polymers, the spacing between adjacent carbon chains tends to decrease over time due to relaxation as they approach an equilibrium state. As a result, permeance goes down and selectivity either remains the same or goes up. The second mechanism is chemical in nature. During use, gaseous species tend to chemiadsorb at active sites in or adjacent to ultramicropores. As a result, the ultramicropores is blocked by the adsorbed species, permeance goes down and selectivity either remains the same or goes up.
Fu, et al. have shown a reduction in the aging effect in CMS membranes after it has been continuously fed a mixed gas of 50% CO2/50% CH4 for a lengthy period of time (Carbon molecular sieve membrane structure-property relationships for four novel 6FDA based polyimide precursors, Journal of Membrane Science, 487, pp 60-73). While this result is interesting, it does not provide a practical solution to the problem of aging in CMS membranes where the gas to be separated is other than 50% CO2/50% CH4. Moreover, Fu, et al. do not propose any pyrolysis atmosphere for solving this problem.
In view of the above-described results and problems existing in CMS membranes, it remains unclear which pyrolysis atmosphere may predictably lead to a more satisfactory performance (such as permeance and selectivity) of a CMS membrane, both initially and over time.
Therefore it is another object of the invention to provide a CMS membrane (and method of making the same and method of using the same) that exhibits a more satisfactory performance than conventional CMS membranes.